1. Field of the Invention
This invention relates generally to projection systems, and more particularly to a novel off axis projection system including a de-centered collimating lens group.
2. Description of the Background Art
Reflective liquid crystal displays (LCDs) provide many advantages over transmissive LCDs, and, therefore, are becoming increasingly more popular for use in projection systems. For example, transmissive displays typically have a limited aperture ratio (i.e., the total area available for light to shine through a pixel) and require pixel fill to separate the pixels, resulting in a pixelated image. The limitations of transmissive displays pose formidable problems in building bright, high resolution displays at a reasonable cost. Reflective LCDs, on the other hand, include an array of highly reflective mirrors manufactured on a standard processed CMOS silicon chip back plane driver, using sub-micron metalization processes recently developed by VLSI process engineers, and do not, therefore, suffer from the limitations of the transmissive displays.
Although superior to transmissive displays in brightness and resolution, reflective displays do pose additional system design problems. For example, FIG. 1 shows a prior art, on-axis projector system 100 to include an illumination source 102, a polarizing beam splitter 104, a color separator 106, a plurality of liquid crystal displays (LCDs) 108(r, g, and b), and projection optics 110. Illumination source 102 generates a source beam of white light and directs the source beam toward polarizing beam splitter 104, which passes one portion of the source beam having a first polarity, and redirects another portion (an illumination beam) of the source beam having a second polarity along a system axis 112, toward color separator 106. Color separator 106 separates the illumination beam into its red, green, and blue components, and directs each of these colored illumination beams to a respective one of LCDs 108(r, g, and b). Each of LCDs 108(r, g, and b) is controlled by a system, e.g., a computer or other video signal source (not shown), and modulates the polarity of selective portions (i.e., pixels) of the colored illumination beams to form colored imaging beams, which are reflected back toward color separator 106. Color separator 106 recombines the colored imaging beams to form a composite imaging beam and directs the composite imaging beam back along system axis 112, toward polarizing beam splitter 104, which passes only the modulated portions of the composite imaging beam to projection optics 110. Projection optics 110 then focuses the modulated portions of the composite imaging beam onto a display surface (not shown).
System axis 112 is defined as a bisector of an angle formed between an illumination beam and an associated imaging beam. As shown in FIG. 1, beam splitting (i.e., color separation) bifurcates system axis 112. Furthermore, optical components which fold the illumination and imaging beam paths (not shown in FIG. 1) cause an associated fold in the system axis.
Because the illumination beams and the imaging beams in system 100 both travel along the same path (system axis 112), projection system 100 is considered an "on-axis" system. On-axis projection systems generally require a polarizing beam splitter such as polarizing beam splitter 104, and, therefore, suffer from the following limitations. First, polarizing beam splitters are highly angular sensitive. Second, polarizing beam splitter 104 must perform both the polarizing function and the analyzing function, and must, therefore, work well for both orthogonal states (S & P) of polarization, thus requiring undesirable manufacturing compromises. Furthermore, polarizing beam splitter 104 introduces a significant path length through glass, which can induce undesirable aberrations in the incident and imaging beams, due to stress induced birefringence. Finally, polarizing beam splitters are very expensive, compared to, for example, polymer based polarizing films.
FIG. 2 shows an off-axis projection system 200 that does not require a polarizing beam splitter. Projection system 200 includes an illumination source 202, a condenser lens 204, a polarizer 206, a reflective LCD 208, an analyzer 210, and a projection lens group 212. Illumination source 202 generates an illumination beam 214 that is focused by condenser lens 204 to pass through polarizer 206, and impinge on LCD 208 at a non-perpendicular angle. LCD 208 modulates illumination beam 214 to form an imaging beam 216, and reflects imaging beam 216 toward projection lens group 212. A system axis 218 bisects the angle formed by illumination beam 214 and imaging beam 216. The angular separation between illumination beam 214 and imaging beam 216 allows for the separation of polarizer 206 and analyzer 210.
Projection lens group 212 focuses imaging beam 216 to project a magnified image of LCD 208 on a display surface 220. Because conventional projection lens groups also magnify the angle of illumination (i.e., the angle between illumination beam 214 and system axis 218), causing undesirable displacement of the projected image, projection lens group 212 is necessarily a custom lens group. Further, the complexity of projection lens group 212 depends on the amount of angular separation between the illumination beam and the imaging beam. In particular, for an angular separation adequate to permit a separate polarizer and analyzer (e.g., 24.degree.), projection lens group 212 would be prohibitively expensive, requiring on the order of 35-40 separate lenses.
What is needed, therefore, is a less complex projection lens system, which allows the angular separation of the illumination beam and the imaging beam, without displacing the projected image (i.e., magnifying the angular separation).